Retardation films, also called retardation plates, are used widely in optical systems. They can either be biaxial where nx′≠ny′≠nz′, or uniaxial where nz′ is equal to either nx′ or ny′. Here we define the direction of wave propagation to be the z-axis. The various symbols nx′, ny′ and nz′ stand for the refractive indices of the material in the various principle directions. Here we define the principle axes of the film as (x′, y′, z′) and the laboratory axes as (x, y, z). Since the films are going to be rotated about the z′-axis, we can let the films principal z-axis and the laboratory z-axis to be identical.
In both uniaxial and biaxial films, the optical retardation for waves polarized in one principal direction, such as the x′-axis, is different from the optical retardation for waves polarized in the orthogonal direction, such as the y′-axis, resulting in modification of the polarization state of any input wave. To be specific, the retardation value of a retardation plate mentioned in this invention is defined as the phase difference between the two orthogonal polarizations and is given by
                    Γ        =                              2            ⁢            π            ⁢                                                  ⁢            d            ⁢                                                  ⁢            Δ            ⁢                                                  ⁢            n                    λ                                    (        1        )            where d is the thickness and Δn=ny′−nx′ is the birefringence of the retardation film, and λ is the wavelength of the input light. If Γ=π, then the retardation plate is a halfwave plate (HWP). If Γ=π/2, it is a quarterwave plate (QWP). Note that the value of nz′ is not a factor for the retardation plates discussed here, since we assume the wave to be propagating in the z-direction. However, nz′ will affect the viewing angle properties of the retardation film. It will have to be considered when both the dispersion and viewing angle have to be optimized.
Retardation films or plates have many applications such as in polarization manipulation and in phase compensation. Of all the retardation films, the halfwave plate and quarterwave plate are the most often used. In display engineering, they are used, for example, in viewing angle enhancement and for dispersion compensation [1]. In projection systems, QWP and HWP are used in polarization conversion optics and in skew ray compensation [2,3]. In all applications, the HWP and QWP should work well over the whole visible spectrum (400˜700 nm). However, conventional HWP and QWP using uniaxial or biaxial retardation films have strong wavelength dependence. As well, their angular dependences are not totally desirable, namely, the retardation changes as the beam propagation direction changes.
Various methods have been proposed to extend the wavelength range of retardation films [4, 5]. Several systems have been proposed making use of new materials (Zhu and Wu, U.S. Pat. No. 6,922,221: Broadband Quarter-Wave Film Device Including in Combination a Chromatic Half-Wave Film and a TN-LC Polymeric Film; Verrall, Ward, Hanmer, and Coates, U.S. Pat. No. 6,544,605: Combination of Optical Elements).
In this invention, we provide a new broadband film design and method of making such films, making use of commercially available wavelength dispersive films. The present invention combines such films to make broadband (achromatic) retardation films. By making and using films according to the present invention, we can provide very broad broadband QWP and HWP films using commercial uniaxial or biaxial retardation films. Very importantly, these QWP and HWP films show negligible wavelength dependence, even at large viewing angles. In addition, the present invention can also be extended to cover retardation films with any targeted dispersion properties. For example, it can have a dispersion that matches that of the birefringence ΔnLC of the liquid crystal material. Thus, full compensation can be achieved for all wavelengths.